In such cases, I generally checkout main, do the side quest, and rebase my original branch on top of it. Then, I can just continue without waiting for the side quest to be merged. Depending on the situation, I may make a separate PR for the side quest, but I don't have to wait for it to get merged.

Alternatively, if you don't like rebasing, you can merge the side quest branch into your project branch instead.

Either way, you don't have to wait for the side quest to get merged.